The invention relates to a device for receiving radio call messages, preferably on board ships, which device comprises a number of receivers for different frequencies and means for processing the messages, the output of each receiver being connected to the input of a shift register of which the output signal, having the value "1" or "0," is fed to the input of an identifier for determining the pertinence of the messages to the relevant ship.
A device of this type for common use is known in principle.
Irrespective of their sort or destination, call messages can be transmitted by various transmitters on said frequencies, predetermined for transmitting call messages. Consequently a certain ship that does not want to miss any of the messages destined to or of importance for it has to listen in on all the relevant frequencies. Hitherto this has been a difficult problem.
Of course it is possible to tune a separate receiver in to each frequency, and to connect a decoder-processor combination to the output. In this way all the messages are received and judged as to their relevance to the ship, after which the messages that are of importance for the ship are further processed. This method, however, requires much equipment for storing and examining messages that are not relevant to the ship.
Another imaginable method comprises only one receiver, which alternately scans all the relevant frequencies and which identifies a message when it is detected. The alternate scanning of five or six frequencies has to take place within 2 seconds, so that no messages will get lost. This time suffices to have a message on one of the frequencies detected and judged as to its relevance by the decoder-processor combination. The identification of the message detected, however, may require as long as 4.5 seconds, so that there is a chance that messages will get lost. This need not be the case when use is made of a coupled system in which the trasmitters are in connection with one another and arrangements can be made as to the moments when messages are to be transmitted. Such a system, which is used in the Japanese waters, is generally not usable in areas with intensive international communication traffic.